I was of the understanding that only the Apple TV 4k and FireTV 4k could stream Dolby Vision content in addition to some Blu Ray players. I believe the Roku Premiere and Ultra are HDR10 only. Maybe I'm operating on old information?
My pick is the ATV4k, but I'm heavily invested in the Apple Ecosystem. I've used a Fire Stick as well and like it just the same, but don't have the same investment as in Apple kit/media.